1. Field of the Invention
The present invention relates to a circuit arrangement for power supply to a transponder, a method for supplying power to a transponder with a first supply voltage, and to a transponder having a circuit arrangement for a power supply.
2. Description of the Background Art
The invention falls within the realm of transponder technology and in particular within the field of contactless communication. Although usable in principle in any communication systems, the present invention and its underlying problem are explained hereafter with reference to transponders for tire pressure control systems and the applications thereof. For a general background of transponder technology, reference is made to the “RFID-Handbuch” (RFID Handbook) by Klaus Finkenzeller, Hanser Verlag, 3rd updated edition, 2002, which was translated into English by John Wiley & Sons.
Transponders are microelectronic components, which are designed to be able to store information and have an antenna array for this purpose, in order to enable contactless reading and modification of the stored information. In the case of transponders, an electromagnetic signal transmitted by the base station is received by the transponder and demodulated. Active, semipassive, and passive transponders are differentiated here depending on the design of their energy supply. In contrast to active transponders, passive transponders do not have their own energy supply, so that the energy necessary in the passive transponder for demodulating and decoding the received electromagnetic signal must be obtained from the electromagnetic signal itself.
Passive transponders are employed primarily in the field of contactless communication for identification (RFID=Radio Frequency Identification) and in tire pressure control systems. Because passive transponders do not have their own energy supply, the energy necessary for supplying the transponder must be removed by absorption modulation from the transmitted electromagnetic wave. In a currently employed passive 150 kHz transponder system, this is realized in the electromagnetic near field by means of inductive coupling. The energy range achieved thereby is within the range of a few centimeters to about 0.5 m and depends inter alia on the specific national HF regulations.
The magnetic portion of an electromagnetic field is utilized in this regard for energy and data transmission, and coils, which represent the inductance of an antenna resonant circuit, are employed as antennas. In order to achieve a sufficient range in passive transponder systems, apart from a uniform alignment or polarization of the antenna coils by the transmitting station and transponder, synchronization of the resonance frequency of the antenna resonant circuit with the working frequency is relevant in addition. The problem arises in practice that frequency-detuning effects change the resonance frequency of the antenna resonant circuit of the transponder. Detuning of the antenna resonant circuit occurs inter alia due to temperature-related drift phenomena at the resonant circuit elements or due to manufacturing-related deviations. Because of this, the resonance frequency of the antenna resonant circuit is no longer trimmed precisely to the transmission frequency, as a result of which the charging current is reduced. It is problematic here that their coupling relationships then worsen, as a result of which even with minor detuning of the resonant circuit, the distance within which reliable energy and data transmission is possible declines considerably.
If the resonance frequency of an antenna resonant circuit is precisely tuned to the frequency of the transmitted or received electromagnetic signal, the impedance of the antenna resonant circuit still consists only of the real ohmic resistance, whereby the maximum charging current and thereby also the maximum field strength are achieved. The maximum possible charging current is then limited solely by the antenna driver and the current supply. Antenna resonant circuits reach the maximum field strengths necessary for data and energy transmission within a tolerance range, which characterizes the range within which the resonance frequency may deviate from the transmission frequency, yet reliable data and energy transmission is still assured. The tolerance range is a gauge for the maximum allowable deviation of the resonance frequency of the antenna resonant circuit from the transmission frequency. If the resonance frequency of the resonant circuit is outside this tolerance range, reliable data and energy transmission is no longer possible. The highest effectiveness in energy transmission is therefore achieved when the antenna resonant circuit of the base station and the antenna resonant circuit of the transponder are operated within the range of the resonance frequency.
The charging of a charging capacitor, which is used for the energy supply to the transponder, occurs via the antenna resonant circuit and rectifier connected downstream to the circuit. The charging current for charging the charging capacitor is highest when the antenna resonant circuit is optimally tuned to the resonance frequency. In fact, in many transponders a rather low charging current is sufficient, because in many transponder applications the charging capacitor is made relatively small. Nevertheless, for some transponder applications, such as, for example, the aforementioned tire pressure control systems, there is need for a relatively large charging capacitor and thereby a high supply voltage.
However, a considerably lower charging current results if the transponder and its antenna resonant circuit are not tuned to the resonance frequency. Because of the lower antenna current, it takes significantly longer to charge the charging capacitor of the transponder. In the extreme case, the capacitor can never be completely charged to its nominal voltage. This has the result that the corresponding transponder does not have its assigned function and thereby is not able to fulfill its task. For this reason, primarily in transponder applications equipped with large charging capacitors, there is a need to tune the antenna resonant circuit to the resonance frequency of a received electromagnetic signal.
The resonance frequency of an antenna resonant circuit of a transponder is typically set by parallel connection of additional capacitive elements to the capacitor of the antenna resonant circuit. The information for the capacitive elements to be connected in parallel is stored in a memory especially provided for this. The problem in tuning the transponder resonant circuit now is to provide as rapidly as possible a voltage to tune the antenna resonant circuit. If the antenna resonant circuit is detuned, however, the charging of the charging capacitor takes very long, so that a voltage for tuning becomes available only very late.
For a general background on such tuning devices to tune an antenna resonant circuit, reference is made to the German patent applications DE 197 55 250 A1 and DE 196 14 455 A1.